First of all, let me try and ease the anger of those people who think that any association of barbecue with a slow cooker is the work of the devil. True barbecue may require a fire, smoke, and lots of time. However, modern convenience has brought the world an appliance that cooks low and slow, and though there is no fire, it still produces tender meat. But before you scoff at the idea remember that many people, for whatever reason, just can't go out back and smoke up some meat.
While slow cookers can do the low and slow part of barbecue, you will not get the authentic flavor of the dry heat and smoke that a backyard smoker will provide. That doesn't mean you can't have great food from a slow cooker. To make meats tender, especially meats like pork roasts, brisket, or ribs you want plenty of cooking time. Slow cooking for 8 hours will generally do the trick, but you will need to make sure that it is fork tender before you take it out of the slow cooker.
Many people attempt to add that smoke flavor to their Crock-Pot barbecue by adding liquid smoke. Liquid smoke is a product made from water, natural hickory smoke flavor, vinegar, molasses, caramel color, and natural flavoring. It adds a smoky flavor to foods and can be used in lots of dishes. Many commercial barbecue sauces (especially the big brand names) have liquid smoke in them.
An alternative to adding liquid smoke is to add foods that contain a smoky flavor. A great alternative is a smoky paprika. Made from smoked and dried peppers, paprika adds a smoky flavor to foods that are mild and delicious. You can also buy a number of smoked chilies and peppers, like chipotle peppers.
Slow cooker barbecue is a very easy process. You can literally throw in your meat of choice, add barbecue sauce, and let it cook all day long. You can also take more extravagant measures. If your meat of choice is fatty, then you will probably want to cook it first on a low-temperature setting with just a hint of seasoning to liquefy most of the fat. Pour this excess fat off before you add any sauce or other ingredients.
When slow cooking meat for sandwiches, like pulled pork or barbecue beef, start with medium sized chunks of meat. This reduces the cooking time but makes the meat easier to shred.
If you haven't been introduced to the real flavor of true barbecue, this is a good place to start.